Membrane Transport

Question 1

Lipid bilayers are the most permeable to molecules that are …
A)lipid bilayers are not permeable to any molecules
B)small and hydrophilic
C)small and hydrophobic
D)large and hydrophobic
E)large and hydrophilic

Answer 1

C)small and hydrophobic

Question 2

Which of the following statements about membranes is incorrect?
A) transporters undergo a conformational change during transport
B) both channels and pores are selective for specific molecules/ions
C) both channels and transporters are capable of active transport
D) Passive transport always occurs down an energy gradient
E) Pores tend to be composed of beta- strands, while channels are composed of alpha- helices.

Answer 2

C) both channels and transporters are capable of active transport
Transporters undergo conformational changes and both channels and pores are selective for specific molecules or ions. Passive transport occurs without the use of ATP so molecules must always move in an energetically favorable direction. Finally, pores are usually formed from beta-strands and channels are composed from alfa-helices. The only false statement is C, because channels are not capable of active transport since they do not undergo conformational changes.

Question 3

You have discovered a receptor that is able to transport both galactose and Na+ into the cell without using ATP. In this case, galactose is moving against its concentration gradient and Na+ is moving with its concentration gradient. This is an example of…
A)active transport
B) passive transport
C) none of the answers are correct
D) secondary passive transport
E)secondary active transport

Answer 3

E)secondary active transport

Question 4

Which of these processes would require the most ATP?
A) transport of sodium into a cell
B) free diffusion of estrogen across a lipid bilayers
C) free diffusion of glucose across a lipid bilayer
D) antiport of sodium into the cell and potassium to the outside of the cell
E) antiport of sodium to the outside of the cell and potassium into the cell.

Answer 4

E) antiport of sodium to the outside of the cell and potassium into the cell.

Question 5

Compare and construct passive vs. active transport and the mechanism of each

Answer 5

Passive transport occurs without every input
Active transport requieres energy input
Small hydrophobic uncharged molecules can pass through the membranes
Ions and polar molecules require protein-mediated (facilitated) transport

Question 6

Which molecule could most easily traverse a synthetic lipid bilayer?
A) carbon monoxide
B)carbon dioxide
C)water
D)glucose
E)these can traverse a lipid bilayer equally

Answer 6

B)carbon dioxide

Question 7

What are the method of facilitated transport?

Answer 7

-Carrier molecules form a hydrophobic shield around polar molecules (often ions)
- Pores/ channels are always passive, are selective, may be gated, there’s no conformational change during transport, are fast
- Transporters are slower, have conformational changes, can be passive or active, are specific and may be regulated

Question 8

Explain transport molecules in depth and explain the modes of action

Answer 8

Transporter proteins can move multiple ligands through conformational changes
- uniport: one direction
Ex: glucose transporter
- symport: 2 molecules, same direction
Antiport: 2 molecules, opposite direction

Question 9

Discuss proton pumping by bacteriorhodopsin

Answer 9

Small changes in protein or retinal can change the color of the light absorbed.
1. Conformational changes in retinal as the proton comes in and out. At ground state, all-trans retinal protonated.
2. Absorb light, adopt cis conformation, release H+
3. Conformational changes, take up a new H+
4. Back to ground state. Retinal is trans and protonated

Question 10

Describe the asymmetry of ionic gradients (Na+ and K+) across the plasma membrane

Answer 10

Na, K-ATPase changes conformation as it pumps ions (antiport, active transport)
1. Na+ binds
2. ATP binds
3. ATP hydrolysis
4. Conformational change. Na+ is released
5. Dephosphorilated K+ binds
6. Conformational change. K+ is released
7. Process starts again
Na+ outside: 150, inside: 5-15}
K+ outside: 4, inside 140

A glucose transporter transports glucose and Na+ into the intestinal cell and another glucose transporter takes it outside the cell. The Na+ is used for the Na+, K-ATPase